Recording digital data on disks and tapes in accomplished by passing a write current through a magnetic transducer to align the magnetic domain of the medium in one of two opposing directions. To achieve high data densities on the medium, a non-return to zero inverting on the ones (NRZI) form of encoding is applied to the data stream to produce the write current. An NRZI encoded signal will maintain its present polarity for each logical zero bit in the data stream, and invert its polarity at the start of each logical one bit. The magnetic domain of the medium follows the write current by orienting in one direction when the write current has a positive polarity, and in the opposite direction when the write current has a negative polarity.
NRZI encoding has a limitation in tape drive and disk drive applications when presented with long strings of logical zero bits. During these long strings, the write current remains at the same polarity. This causes long regions in the data tracks to be magnetically oriented in the same direction. Long regions of uniform magnetic orientation sometimes cause a magneto-resistive read sensor to saturate, resulting in data errors.
The Full-Cell Write Equalization method was developed to prevent read sensor saturation. Full-Cell Write Equalization method briefly inverts the write current polarity one or more times during long string of zeros. These brief polarity inversions are called equalization pulses. For a string of N consecutive logical zero bits, the Full-Cell Write Equalization method generates N-X equalization pulses. The value of X is typically equal to d, the minimum number of zeros between adjacent ones for a (d,k) run length-limited modulation code. These equalization pulses are typically centered in time during the writing of the logical zero bits, starting with the first logical zero bit in the string. Other encoding methods space the equalization pulses at equal intervals between the NRZI transitions caused by logical one bits that lead and trail the string of consecutive logical zero bits.
Another problem occurs where data stream encoding causes the write current transitions at high frequencies. The net effect of these high frequency transitions is to shift the apparent position of the leading and trailing transitions as seen by the read sensor. These apparent shifts may sometimes cause problems in the read circuitry. The Write Pre-Compensation method compensates for the apparent shifts seen by offsetting the transition write positions in the opposite direction of the apparent shift. This method is well known in the field of disk drives. Shifts in the apparent position of the data on magnetic medium are sometimes caused by slow rise-times in the write transducers. The Write Pre-Emphasis method applies extra power at the start of a transition to produce a faster rise-time in the leading edge of the transitions. While these two methods reduce the apparent shifts, they require special circuitry to generate the complex write currents.
A common practice in the disk drive and tape drive industry is to read the data from the magnetic medium as it is being written to verify that the correct data is being recorded. For space and alignment purposes, the read sensors and write transducers are usually co-located in the same magnetic head. This close physical proximity, and the wiring connecting the magnetic head to circuit cards, results in cross-talk between the write channel and the read channel. Signals in the write channel are on the order of volts. Signals in the read channel are only on the order of millivolts. Consequently, any time power is applied to the write channel it is picked up in the read channel as unwanted noise.
The Uniform Pulse-Write method partially solves the problem of write signal noise in the read channel by reducing the amount of power in the write signal. This is accomplished by modulating the write current with a duty cycle so that part of the time the write current is not being applied to the write transducer. The frequency of the duty cycle is sufficiently high so that the individual pulses overlap and blend in the magnetic medium. As a result, data written using the Uniform Pulse-Write method has the same read characteristics as data written using a non-pulsed method.
The Pulse-On-Transition method requires even less power than the Uniform Pulse-Write method. Pulse-On-Transition pulses the write current at the same time that the Full-Cell Write Equalization method inverts the write current's polarity. The Pulse-On-Transition method returns the write current to zero at the completion of each pulse. Because some pulses start from a zero write current condition, data written using this method has different read characteristics than data written using the Full-Cell Write Equalization method making the two methods incompatible. Another reason the two methods can have different read characteristics is that a portion of the magnetization in the write transducer may switch so slowly that the magnetization continues to increase as long as the transducer is energized. Therefore, the magnetic state before and after the polarity inversion may be very different for the two methods.
As recording densities increase, the signals from the read sensors will decrease causing a lower signal to noise ratio. To maintain a reasonable signal to noise ratio, the write current induced noise needs to be decreased. A new encoding method is required that produces less write current than the Full-Cell Write Equalization method. At the same time, the data written on magnetic medium using the new encoding method should have the same read characteristics as data written using the Full-Cell Write Equalization method to maintain compatibility with existing disk drives and tape drives.